From 566a9b76b85a920e6fcb7e15f1d8d5d7f46b69ea Mon Sep 17 00:00:00 2001
From: Jeremy Kaplan <jdkaplan@metagram.net>
Date: Wed, 9 Dec 2020 21:51:34 -0800
Subject: [PATCH] Day 10 Part 1

---
 2020/day10/day10.ex | 48 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 48 insertions(+)
 create mode 100644 2020/day10/day10.ex

diff --git a/2020/day10/day10.ex b/2020/day10/day10.ex
new file mode 100644
index 0000000..6f4a67e
--- /dev/null
+++ b/2020/day10/day10.ex
@@ -0,0 +1,48 @@
+defmodule Day10 do
+  defp read_input do
+    Path.expand('input', Path.dirname(__ENV__.file))
+    |> File.read!()
+  end
+
+  defp parse_adapters(text) do
+    joltages = text |> String.trim() |> String.split("\n") |> Enum.map(&String.to_integer/1)
+    joltages ++ [3 + Enum.max(joltages)]
+  end
+
+  defp super_power(adapters, stack = [top | _rest]) do
+    as = Enum.filter(adapters, fn next -> can_stack?(top, next) end)
+
+    if Enum.empty?(as) do
+      stack
+    else
+      next = Enum.min(as)
+      remaining = Enum.reject(adapters, fn a -> a == next end)
+      super_power(remaining, [next | stack])
+    end
+  end
+
+  defp can_stack?(a1, a2) do
+    d = a2 - a1
+    1 <= d and d <= 3
+  end
+
+  defp pairs(enum) do
+    Enum.chunk_every(enum, 2, 1, :discard)
+  end
+
+  def part1 do
+    adapters = read_input() |> parse_adapters()
+
+    counts =
+      super_power(adapters, [0])
+      |> pairs()
+      |> Enum.reduce(%{}, fn [a2, a1], counts ->
+        d = a2 - a1
+        Map.put(counts, d, Map.get(counts, d, 0) + 1)
+      end)
+
+    counts[3] * counts[1]
+  end
+end
+
+Day10.part1() |> IO.inspect()